MR tech here. I once had a minor accident with an angle grinder (well, I've had a few, but this one had side effects) and got several minuscule steel fragments in a finger. I didn't know this had happened, except that once at work (edit: I work on 3T scanners) I could feel them in my finger. They were slightly sore, and very annoying. A few x-rays later I managed to localise them and with the help of a colleague, dug them out with a needle. I never did find them, but we got them out. I have scanned quite a few people with various sorts of shrapnel in them. Only once was it a problem for the patient (moderate pain) but we don't take the scary ones in - intraocular metallic fragments, one deep in a neck etc.